1. Field of the Invention
The present invention relates to the in situ removal of contaminants from subsurface soil and groundwater by the use of a pressure extraction process.
2. Description of Prior Art
Underground soil and water contamination is a serious problem, especially when the contamination can affect underground aquifers that are used for drinking water.
A typical underground soil structure of the type that is readily susceptible to subsurface contamination comprises a porous, unsaturated upper soil layer, called the "vadose zone", which extends downward from the surface of the ground. The vadose zone is often positioned over a layer of water-saturated soil, wherein the water and soil are called a "water-saturated zone". This in turn typically rests on a base of low-permeability material such as clay or bed rock. The base is also called an "aquitard". The water and soil volume encompassed in a zone of water-saturated soil is called a "groundwater unit" and the water in the saturated soil is called "groundwater". The top of the groundwater unit adjacent the vadose zone is called the "groundwater surface" or "water table". A groundwater unit that is sufficiently large to provide a source of drinking water is commonly referred to as a "aquifer". The area of the vadose zone immediately above the groundwater surface is called the "capillary fringe". This is an area of increased water concentration in comparison with the upper regions of the vadose zone.
Vapor and liquid pressure conditions underneath the surface of the ground are similar to those above ground, with underground soil typically acting as a porous framework that restrains but does not prevent vapor and liquid flow. Underground soil does not exert a substantial compressive force on underground vapors and liquids. The air or vapor pressure at the upper surface of a groundwater unit is approximately atmospheric pressure, which is zero psig or approximately 14.7 psia at sea level. Pressure levels increase with depth below the groundwater surface in substantially the same way as hydraulic pressure increases in relation to depth in any body of water.
Underground water usually tends to flow in a particular direction, which is referred to as "downgradient" or "down slope". This is comparable to "downstream" flow for surface water. A downgradient flow of contaminated groundwater causes contaminants associated with the water to be spread to areas other than the area of the initial spill. When the downgradient flow leads to fresh water wells that provide drinking water, contamination of population centers can result.
A number of substances, including organic compounds and inorganic compounds such as dissolved metals, can cause contamination. One of the more common groups of chemicals that can cause toxic contamination is called "volatile organic compounds" or VOC's. These include petroleum hydrocarbons (e.g., gasoline and other fuels) and chlorinated and hydrogenated aliphatics (e.g., solvents, degreasing agents and cleaning solutions). VOC's and other contaminants spilled on the surface of the ground tend to migrate downwardly through the soil under the influence of gravity and infiltration of water until the contaminants degrade or dissipate or until they come in contact with the groundwater surface. Petroleum hydrocarbons, which are generally of lower density than water, tend to spread out or float on the groundwater surface or become dissolved in the upper regions of the groundwater. Non-aqueous phase contaminants floating on groundwater are called "free product". VOC's and other contaminants that have a higher density than water, such as many chlorinated and hydrogenated aliphatics, tend to sink in the groundwater, becoming dissipated or dissolved in the groundwater as they penetrate through the groundwater.
VOC's and other contaminants contaminate the ground in several ways. They become sorbed on or in the grains of soil they contact; they are held in the soil structure in the voids between the grains of soil; they volatize and are held in the soil structure in vapor form; they dissolve in the groundwater; and they float on the groundwater or sink in the groundwater.
As additional fresh water infiltrates down through the soil from the surface of the ground, contaminants that had been suspended in the soil structure are contacted by the water and gradually become dissolved or entrained in the water. This provides a continuing and persistent source of contamination. Accordingly, it is desirable to remediate contamination in the vadose zone as well as on or in the groundwater.
A number of techniques have been used for removal of subsurface soil contaminants and for remediation of the affected soil and groundwater. One technique is excavation of the contaminated site and removal and treatment of the affected soil. This, however, can be extremely costly or infeasible with increasing depths below grade surface.
A number of techniques are used to treat the soil or groundwater in place (in situ). One method includes vacuum extraction of free product contaminants floating on the groundwater surface. Vacuum extraction techniques also are used to volatize contaminants and remove vapors from the vadose zone soil structure above the groundwater.
Vacuum extraction processes which reduce the soil vapor pressure in proximity to the groundwater surface have been shown to cause upwelling of the groundwater surface proportional to the level of vacuum applied. The upwelling frequently saturates vadose zone and capillary zone soils, and makes that submerged area less accessible to volatilization and aerobic degradation of contaminants.
A technique known as "sparging" is frequently used in conjunction with soil vacuum extraction and involves injecting oxygen-containing gas below the groundwater surface and vacuum extraction of gas from vadose zone soils.
Another contamination removal process that has had some success employs vacuum extraction for removal of volatile contaminants from the vadose zone and contaminated groundwater. One system employing this process employs a high-vacuum extraction well comprising a perforated outer well casing that extends below the groundwater surface and a high-vacuum interior pipe that draws liquid and vapor into the outer casing from above and below the groundwater surface and expels the liquid and vapor by vacuum extraction out of the inner pipe. This process has been commercialized by Xerox Corporation.
There are some drawbacks to this system. The use of a high level of vacuum to extract vapors from the vadose zone and from the capillary fringe creates a reduced pressure zone outside of the well casing above the groundwater surface. This reduced pressure can change the contours of the groundwater surface adjacent the well casing in the manner described above for vapor vacuum extraction systems. The vacuum extraction of groundwater may depress the groundwater surface in the immediate proximity to the well casing, but the reduced pressure in the vadose zone above the groundwater surface at least partially offsets this effect by urging the groundwater upwardly. There is a concern that the vacuum that induces upwelling could, under some circumstances, exceed the depression effect caused by groundwater extraction, in which case the groundwater would actually rise to form a raised collar or bulge in the groundwater surface around the well casing (see FIG. 3). The outer surface of a raised collar would slope away from the well casing and would thus urge free product floating on the groundwater outside of the collar to flow away from the extraction well. Raising the groundwater level also submerges an area of the vadose zone and capillary fringe and makes that submerged area less accessible to volatilization of contaminants.
An object of the present invention is to provide a better method for in situ removal of subsurface contaminants.